1. Field of the Invention
This document relates to an electric motor-driven compressor for a vehicle. More particularly, this document relates to an electric motor-driven compressor having a structure capable of improving the performance of an electric motor of the electric motor-driven compressor for the vehicle.
2. Description of the Related Art
Recently, an electric vehicle or a hybrid vehicle employing both fossil fuel and electricity as a driving source has been spotlighted according to the policy of low-pollution and high fuel efficiency policy derived from depletion of the fossil fuel and the environmental pollution, so the researches on the above vehicles have been actively performed.
The hybrid vehicle or the electric vehicle obtains driving power through an electric motor. Accordingly, an electric compressor has been extensively used for an air conditioning system of a vehicle instead of a mechanical compressor.
The electric compressor includes an electric motor to convert electrical energy into dynamic energy and an inverter to control the rotation of the electric motor. The electric motor of the electric compressor typically includes a cylindrical rotor and a stator surrounding the rotor and including the coil wound around the stator. A distributed winding scheme or a concentrated winding scheme may be employed according to the winding scheme of the coil.
In the electric compressor, as current flows through a coil by power supplied from the inverter, the rotor of the electric motor is rotated, and the rotational force of the rotor is transferred to a rotational shaft. In addition, a mechanical unit, which has received the dynamic energy from the rotational shaft, reciprocates to compress the coolant.
When comparing with the conventional mechanical compressor, the performance to compress the coolant of the electric compressor is degraded. This is because the electric compressor represents the weak driving force generated by the electric motor, great vibration, and the degraded control performance of the inverter when comparing with the mechanical compressor driven by using the rotational force of an engine.
In addition, since the electric motor represents low efficiency, the electricity supplied to the vehicle may be wasted.
Examples of factors causing the vibration of the electric motor and reducing the precision of the electric motor are cogging torque and torque ripples caused by the interaction between a permanent magnet and a slot. The cogging torque and the torque ripples inevitably exist in the motor having slots.
The cogging torque is a force applied to a motor system in a radial direction so that the motor system intends to move to the position representing the minimum magnetic energy, that is, to an equilibrium state. The cogging torque is irregular torque generated due to the mutual relation of a pole of a permanent magnet and a slot regardless of load current and exerts a great influence on the control and precision of a motor.
In order to reduce the cogging torque, a scheme of reducing the cogging torque by properly combining the number of magnetic poles of a permanent magnet with the number of slots is generally known in the art.
However, in the fields of an electric motor used in a compressor for a hybrid vehicle, researches and studies have been rarely carried out with respect to the optimal ratio of the number of magnetic poles to the number of slots, or the structure and the design for reducing cogging torque and torque ripples.